Answer: Dwarf varieties of red cedar (Juniperus species) are very drought tolerant, but any plant needs moisture to survive, as you've learned. A product like Terra Sorb, which Burpee carries, when mixed with soil, helps hold moisture where plants need it.
Good perennials to try are coreopsis, gazania, and painted daisies. For insulation against temperature extremes in winter, surround the planters with straw bales or tie styrofoam packing slabs around the planters, and disguise them with evergreen boughs or the like. Hope this helps!
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